It is desired to put polymer electrolyte fuel cells using a polymer electrolyte membrane into practical use as early as possible as the power source for households, power source for electric vehicles, and power source for mobile devices such as cell phones and notebook personal computers.
Polymer electrolyte fuel cells (hereinafter referred to as simply “fuel cells”) have at least one cell including a membrane electrode assembly (hereinafter referred to as an “MEA”) and a pair of separators. The MEA includes a polymer electrolyte membrane, an anode, and a cathode. The anode and the cathode each comprise a catalyst layer and a gas diffusion layer. The anode and the cathode are bonded to the main faces of the polymer electrolyte membrane so that they face each other with the polymer electrolyte membrane therebetween. Further, both faces of the MEA in the thickness direction thereof are sandwiched between the pair of separators.
In such a fuel cell, a fuel such as hydrogen and an oxidant such as air are supplied to the anode and the cathode, respectively, to generate power.
When a fuel cell is used as a power source, except when used for a small device such as a cell phone, the fuel cell is combined with air and fuel supply means and a power generation control means to form a fuel cell system. In such a fuel cell system, the fuel cell is specifically referred to as a fuel cell stack. The supply means supply a fuel to the anode and air to the cathode. For the supply means, for example, a blower or pump is used. The power generation control means controls the amount of power generated by the fuel cell stack, the temperature of the fuel cell stack, the supply of air and fuel, the stopping of the supply thereof, etc.
However, practical utilization of fuel cells has several problems to be solved.
One problem to be solved is long-term life characteristics.
The output of fuel cells gradually decreases with increasing power generation time. Fuel cells are required to maintain their output for a total of 40000 hours or more when used as the power source for households, and for a total of 5000 hours or more when used as the power source for mobile devices. At present, their life characteristics do not meet such requirements.
The output decrease is attributed to several factors, one of which is deterioration of air diffusion on the cathode side.
At the cathode, water is produced by electrode reaction as power is generated. In addition, when the fuel is hydrogen, both air supplied to the cathode and hydrogen supplied to the anode usually contain water, because they are humidified by a humidifier to suppress dry-out of the MEA and thus deterioration of the proton conductivity of the polymer electrolyte membrane. Thus, the air used as the oxidant also brings water to the cathode. Also, when the fuel is methanol, water contained in a methanol aqueous solution supplied to the anode moves to the cathode through the polymer electrolyte membrane. In this way, in either case, water accumulates in the cathode during power generation.
Such water accumulated in the cathode during power generation can cause deterioration of air diffusion in the cathode if it is not sufficiently removed.
In view of such problems, for example, Japanese Laid-Open Patent Publication No. 2003-031254 proposes a method for operating a fuel cell system in which after the power generation of the fuel cell is stopped, a gas for drying is supplied to the cathode for a predetermined period of time. This method intends to remove water remaining in the cathode with the dry gas having a low relative humidity.
Also, for example, Japanese Laid-Open Patent Publication No. 2007-042445 proposes another method for operating a fuel cell system. In this method, immediately before the power generation of the fuel cell is stopped, power is generated such that a large amount of water remains in the cathode, and after the power generation is stopped, a purge is performed with a gas of low humidity.
In order to prevent the MEA from becoming too dry as a result of a purge with a low humidity gas, this method intends to fully hydrate the MEA before stopping the power generation and remove only large droplets remaining in the air flow channel.
However, according to these operation methods, it is difficult to provide satisfactory long-term life characteristics required of fuel cells. That is, in these methods, a purge with a low humidity gas is performed whenever the power generation is stopped, regardless of the amount of water remaining in the MEA or cathode. These methods make the MEA too dry when the amount of water remaining in the MEA is small. Since fuel cell stacks are usually not sealed gas-tightly, the water accumulated in the cathode gradually evaporates and dissipates from the cells while the power generation is stopped. After the power generation is stopped for a certain period of time, the amount of water remaining in the MEA is small, and in such state, the amount of water accumulated in the cathode is not so large as to require drying by a purge. In such cases, these methods may make the MEA too dry.
When the polymer electrolyte membrane dries and its proton conductivity lowers, the output decreases temporarily until the electrolyte membrane is hydrated again, and, in addition, the electrode potentials vary significantly due to a large overvoltage. This can result in deterioration of the catalyst and electrolyte. Further, the use of air for each purge may further promote the deterioration of the catalyst and electrolyte since the cathode potential becomes high for an increased period of time.
Accordingly, although these operation methods may suppress the accumulation of water in the cathode, they will promote deterioration for other reasons.
The invention is achieved in view of the problems as discussed above, and an object of the invention is to provide a fuel cell system with excellent long-term life characteristics.